Pumped vertical drain

ABSTRACT

A vertical drain for consolidating soil, the drain comprising an elongate surrounding filter jacket having a proximal end and a distal end; and a suction tube having at least one suction aperture located within the jacket, wherein, in use, a suction pump connected to the suction tube is operable to pump flowable material through the suction tube.

TECHNICAL FIELD

THE PRESENT INVENTION relates to vertical drains and more particularly to a vertical drain product, a method of installing a vertical drain, and a method of using a vertical drain.

BACKGROUND

Before a construction can be developed in an area, it is necessary for the soil beneath the construction area to be consolidated. The soil consolidation process improves the load bearing capacity and stability of the soil. This process is particularly important when the construction is to be located on compressible soils. Compressible soils such as clay are characterised by having a large pore space which accommodates water, referred to as “pore water” herein, and have a relatively weak structure. If a heavy load, such as a construction, is placed on such soil, it cannot support the increased load and the pore water is slowly squeezed out from the pores. This process causes the soil to settle, which can damage or destroy the construction.

There are several well-known methods of accelerating the consolidation of soil before construction takes place. One such method is installing vertical drains to dewater the soil. The vertical drains comprise vertical bores, the bore extending down into the ground to be consolidated. An embankment load (also known as an overburden) such as a large volume of sand is added to the top surface of the soil. The pressure of the overburden increases the pressure of the pore water, causing it to escape and permeate through the walls of the vertical drains. Early vertical drains were vertical bores filled with sand. The overburden forces water into the sand drains thereby consolidating the ground.

Sand drains have been replaced with so-called “pre-fabricated vertical drains” (PVDs). PVDs typically comprise an elongate plastic corrugated core surrounded by a filter jacket. Water is free to pass through the filter jacket into the corrugations of the core. The water in the vertical drain is forced up through a series of channels in the core and to the surface by the pressure of the surcharge load on the ground above. Alternatively, the water can be pumped up the vertical drain using a pumping system.

If the soil is impermeable and vertical drains are not installed, the water takes a very long time to leave the soil. Vertical drains shorten the distance the water has to travel to leave the soil, thereby accelerating the consolidation process. Vertical drains used in this manner can reduce the time to achieve a given level of consolidation from decades to years, depending upon the soil condition, the spacing of the drains and the weight of surcharge load.

When using PVDs, the rate of consolidation begins to slow after 60% consolidation has been achieved. The waiting period necessary to achieve further consolidation is lengthy and therefore, in most circumstances, impractical. In order to speed up the consolidation process, it is known to increase the surcharge load on the ground being consolidated. However, there are associated problems with this solution including that the surcharge load can become unstable, there may be a shortage of surcharge material and the additional time and cost considerations for increasing the surcharge load.

This invention seeks to provide an improved vertical drain which accelerates the consolidation process.

SUMMARY

One aspect of the present invention provides a vertical drain for consolidating soil, the drain comprising:

an elongate surrounding filter jacket having a proximal end and a distal end; and

a suction tube having at least one suction aperture located within the jacket, wherein, in use, a suction pump connected to the suction tube is operable to pump fluid and/or flowable material through the suction tube.

Preferably, the vertical drain further may comprise an anchor plate at the distal end of the filter jacket.

Conveniently, the at least one suction aperture may be located substantially at the distal end of the jacket.

Advantageously, the suction apertures may be located at least 0.25 m and/or 0.50 m from the proximal end of the jacket.

Preferably, the at least one suction aperture may comprise an open end of the suction tube.

Conveniently, the at least one suction aperture may comprise a perforated portion of the suction tube.

Advantageously, the suction tube may be perforated at substantially 100 mm intervals along at least a part of the length of the tube.

Preferably, there may be an opening in the filter jacket to receive a length of the suction tube.

Conveniently, there may be an opening in the filter jacket to receive a length of the suction tube and a length of the suction tube may be perforated to provide the at least one suction aperture and may be located inside the filter jacket, and a length of the suction tube without a suction aperture may pass through the opening in the filter jacket and may be located outside the filter jacket.

Advantageously, the suction tube may be perforated not more than 1 m from the distal end of the filter jacket.

Preferably, the vertical drains may be in an array.

Conveniently, the vertical drain may further comprise a draincore within the filter jacket to provide a support structure.

Advantageously, the drain core and/or filter jacket and/or suction tube may be integrally formed and/or be separate from each other and/or be connected to each other.

Another aspect of the present invention provides a method of consolidating soil, the method comprising:

installing a vertical drain into the soil to be consolidated, the vertical drain comprising an elongate surrounding filter jacket having a proximal end and a distal end and a suction tube having at least one suction aperture located within the jacket; and

operating a suction pump connected to the suction tube to pump fluid and/or flowable material that permeates through the filter jacket from the surrounding soil through the suction tube.

Preferably, the at least one suction aperture may be located substantially at the distal end of the filter jacket.

Conveniently, the method may further comprise forming the vertical drain.

Advantageously, forming the vertical drain may involve securing at least a portion of the suction tube to the inside of the filter jacket, such that at least one suction aperture is located near to the distal end of the filter jacket.

Preferably, forming the vertical drain may involve securing at least a portion of the suction tube such that suction apertures are located at least 0.25 m from the proximal end of the jacket.

Conveniently, forming the vertical drain may involve securing at least a portion of the suction tube such that suction apertures are located at least 0.5 m from the proximal end of the jacket.

Advantageously, the method may further comprise adding an overburden layer to the soil and/or the use of electro-osmosis and/or the use of vacuum consolidation to increase the rate of water released from the soil.

Preferably, the method may further comprise pumping the fluid into a horizontal drain.

A further aspect of the present invention provides a method of installing a perforated suction tube into a vertical drain, the vertical drain comprising an elongate surrounding filter jacket having a proximal end and a distal end, and the method comprising:

forming an opening in the filter jacket;

inserting at least a portion of the suction tube through the opening;

subsequent to inserting at least a portion of the suction tube, sealing the opening in the filter jacket; and

subsequent to inserting and securing the suction tube, installing the vertical drain into an area of soil to be consolidated.

Conveniently, the method may further comprise:

-   -   perforating a tube to form the perforated suction tube, the tube         having two ends; and     -   sealing one end of the suction tube.

Advantageously, installing the perforated suction tube may further comprise securing at least a portion of the suction tube inside the jacket such that at least one suction aperture is located substantially at the distal end of the filter jacket.

Preferably, installing the perforated suction tube may further comprise securing at least a portion of the suction tube inside the jacket such that suction apertures are located at least 0.25 m and/or 0.50 m from the proximal end of the jacket.

Conveniently, the vertical drain may further comprise a drain core and the method may further comprise securing the tube to the drain core.

Advantageously, the sealed end of the perforated tube may be secured to the drain core.

Preferably, the perforated tube may be secured to the drain core by stapling.

Conveniently, the method may further comprise installing the vertical drain with a mandrel.

Advantageously, the method may further comprise installing an anchor plate substantially at the distal end of the filter jacket.

Preferably, the suction tube may be perforated at substantially 100 mm intervals along at least a part of the length of the tube.

Conveniently, the suction tube may be perforated not more than 1 m from the distal end of the filter jacket.

Advantageously, the opening in the filter jacket may be formed substantially 1.5 m from the distal end of the filter jacket.

Preferably, the opening in the filter jacket is sealed with tape.

The present invention further provides a vertical drain, a method of installing a vertical drain and a method of using a vertical drain as claimed.

In order that the present invention may be readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-section of an array of installed vertical drains for consolidating soil from prior art;

FIG. 2 shows schematic cross-section of an array of installed vertical drains for consolidating soil that are connected to a pump from prior art;

FIG. 3 shows schematic cross-section of an array of installed vertical drains suitable for use with a prior art vacuum consolidation method for consolidating soil from prior art;

FIG. 4 shows a prior art vertical drain;

FIG. 5 shows a schematic cross-section of a vertical drain embodying the present invention;

FIG. 6 shows a schematic cross-section of an installed vertical drain embodying the present invention;

FIG. 7 shows a schematic cross-section of another installed vertical drain embodying the present invention;

FIG. 8 shows a plan view of a vertical drain embodying the present invention; and

FIG. 9 shows a schematic cross-section of an array of installed vertical drains embodying the present invention.

DETAILED DESCRIPTION

FIG. 1 shows an array of three prior art vertical drains 100 for consolidating compressible soil. In FIG. 1 three layers of soil are shown: a top embankment layer 101 providing an overburden; a middle layer of clay 102 to be consolidated; and a bottom layer 103 usually of impermeable stiff clay. The vertical drains 100 comprise a permeable filter jacket 105 with a proximal end 104 and a distal end 106. The drains are arranged such that the proximal ends 104 of the drains 100 are in the embankment layer 101 and the distal ends 106 of the drains 100 are in the bottom layer 103. An anchor plate 110 is fixed to the vertical drains 100 at the distal end 106 of the vertical drain 100.

The soil 102 cannot support the increased load of the embankment and the pore water is squeezed out from the pores and into the vertical drains 100. In the prior art configuration of FIG. 1, the water is collected in the vertical drains 100 and forced into both the bottom layer 103 and the embankment layer 101 by the pressure of the overburden.

FIG. 2 shows an array of three prior art vertical drains 100. In this prior art example, the vertical drains 100 comprise the same proximal end 104, distal end 106 and filter jacket 105 as in FIG. 1. A cap 108 is fixed to the vertical drain 100 at the proximal end 104, the cap 108 connecting the proximal end 104 of the filter jacket 105 to a pumping system comprising pump tubing 107 and a pump 109.

In the FIG. 2 prior art example, there is one layer of clay soil 102 to be consolidated. The embankment load 101 is placed on top of the clay layer 102. The clay layer 102 cannot support the embankment 101 and the pore water is squeezed out from the pores and into the vertical drains 100. In this example, the water collected in the vertical drains 100 is then pumped out of the vertical drains 100 by the pumping system.

FIG. 3 shows a prior art vacuum consolidation method. In FIG. 3, there is one layer of clay soil 102. The vertical drains 100 in this figure are similar to those in FIG. 1, comprising a permeable filter jacket 105, a proximal end 104 and a distal end 106, and are arranged such that the proximal ends 104 of the drains 100 are near the surface of the clay soil 102. In this example, the pumping system further comprises an impermeable sheet 111 or an impermeable layer of soil 111 at ground level.

Instead of applying and then removing the overburden of the previous prior art examples, a surcharge is created in the clay soil 102 using a vacuum pump. This surcharge simulates the overburden load. Again, the clay soil 102 cannot support the simulated embankment load and the water is pumped from the proximal end 104 of the vertical drains 100 using the pumping system shown.

FIG. 4 shows a prior art vertical drain 100 with a connecting pipe 107, a cap 108 and a permeable filter jacket 105. The connecting pipe 107 terminates at the proximal end 104 of the permeable filter jacket 105.

The prior art vertical drains are normally expected to drain only into the top sand layer or facilitate suction only from the proximal end of the vertical drain.

FIG. 5 shows a vertical drain 1 embodying the present invention. In this embodiment, the vertical drain 1 comprises an elongate surrounding filter jacket 2 having a distal end 3, a proximal end 4 and a suction tube 5. The suction tube 5 has two ends 6, 7 and at least one suction aperture located within the jacket. In this embodiment, the suction aperture is an open end 6 of the suction tube 5. A suction pump 9 is connected to the suction tube 5 with a section of piping 8. When the vertical drain 1 is in use, the suction pump 9 connected to the suction tube 5 is operable to pump fluid and/or flowable material through the suction tube 5.

Preferably, suction apertures are located substantially at the distal end 3 of the jacket 2.

FIG. 6 shows a second vertical drain 1 embodying the present invention. In this embodiment, the vertical drain 1 further comprises an anchor plate 10 at the distal end 3 of the filter jacket 2 to secure the vertical drain 1. The vertical drain 1 also comprises a drain core 11 to provide a separation volume within the filter jacket 2, spacing the walls of the filter jacket 2 apart so that pore water may collect within the jacket 2. The core 11 may also provide a support structure within the jacket 2.

In FIG. 6, the vertical drain 1 is installed in soil to be consolidated. In this embodiment, the suction tube 5 extends from the proximal end 4 of the filter jacket 2 to the distal end 3 of the filter jacket 2. In this embodiment, the at least one suction aperture comprises a perforated portion of the suction tube 5 with perforations 13 along at least a part of the length of the suction tube 5. In some embodiments the perforations 13 may be arranged to be in pairs on opposing sides of the walls of the suction tube 5.

In a preferred embodiment, the drain core 11 is corrugated or finned (a series of equi-spaced projections or ribs) and the corrugations, fins, ribs or projections define between themselves a plurality of elongate channels 12 in the core 11. In other embodiments, another structure may be used, for example a wire mesh.

Preferably, the suction tube 5 is perforated at substantially 100 mm intervals along at least a part of the length of the tube 5. In another preferred embodiment, the suction tube 5 is perforated not more than 1 m from the distal end 3 of the filter jacket 2.

The FIG. 6 embodiment improves on the open-ended suction tube 5 of FIG. 5. If fine clay or soil particles pass through the filter jacket 2 and accumulate at the distal end 3, then this can result in the only opening available becoming clogged or blocked. Additionally, if the suction tube opening is the same internal diameter as the suction tube 5, then particles can be sucked into the tube 5 and become stuck somewhere along the tube 5. Thus, a perforation of smaller size than the tube diameter is preferred.

The perforation diameter is made substantially smaller than the tube internal diameter so that any particle that can get into the system will not get stuck within the tube. For this reason, the suction tube end 6 (which can form the end of the suction tube 5 in the FIG. 5 embodiment) is sealed so that the only openings to the tube 5 are the perforations 13.

In some embodiments, there is a length of 1-2 m of the suction tube 5 within the filter jacket 2. The perforations 13 are preferably remote from the proximal end 4 of the filter jacket 2 and any opening that there might be in the filter jacket 2 for receiving a length of the suction tube. In this manner, there is no risk of exposing the perforations 13 in the suction tube 5 to the clay or surrounding soil during installation of the vertical drain 1. Thus, the last one metre of the suction tube 5 adjacent to the distal end 3 of the filter jacket 2 is safely perforated. The perforations 13 are preferably at least 0.5 m away from the opening if the suction tube 5 enters the filter jacket 2 at 1.5 m from the distal end 3 of the filter jacket 2.

There is a balance between the number of perforations 13 and maintaining the structural integrity of the suction tube wall. Too few perforations increase the risk of all the perforations 13 becoming clogged whilst too many perforations 13 weaken the tensile strength of the suction tube wall. At a 100 mm spacing there are ten pairs of perforations 13 safely away from the suction tube insertion point along the filter jacket 2 and these perforations 13 do not have a substantial effect on the strength of the tube 5.

As the fluid level decreases to the level of the perforations 13 (which are deep in the soil to be consolidated), adequate soil consolidation has taken place, especially as the vertical drain 1 is normally driven somewhat into the stiff layer of soil below the soil to be consolidated.

FIG. 7 shows a cross-section of a vertical drain 1 embodying the present invention. FIG. 7 shows the filter jacket 2 surrounding the drain core 11. In other embodiments, the drain core 11 may be of any structure and the vertical drain 1 may comprise any number of suction tubes 5. In another embodiment, the single vertical drain 1 may comprise a plurality of suction tubes 5.

The diameter of the suction tube 5 is significantly larger than the size of the core channels 12. Preferably, the tube diameter is in the order of 10 mm and the core channels 12 are in the region of 2-3 mm wide. Conveniently, the tube 5 sits between and alongside the core 11 and the filter jacket 2.

FIG. 8 shows one example of an array of vertical drains 1 installed in soil to be consolidated. The array may be of any size, be of any shape and include any number of vertical drains 1.

The embodiments described allow the fluid to be extracted from the distal end 3 of the filter jacket 2, which decreases the consolidation time necessary to achieve a predetermined level of consolidation.

One method of consolidating soil according to the present invention comprises installing the vertical drain 1 into the soil to be consolidated, the vertical drain 1 comprising an elongate surrounding filter jacket 2 having a proximal end 4 and a distal end 3 and a suction tube 5 having at least one suction aperture located within the jacket 2; and operating the suction pump 9 connected to the suction tube 5 to pump fluid and/or flowable material that permeates through the filter jacket 2 from the surrounding soil through the suction tube 5.

In a preferred embodiment, the at least one suction aperture is located substantially at the distal end 3 of the filter jacket 2.

In some embodiments, the method comprises forming the vertical drain 1, which may further comprise securing the suction tube 5 to the inside of the filter jacket 2 such that at least one suction aperture is located substantially at the distal end 3 of the filter jacket 2. In a preferred embodiment, the suction tube 5 is secured to the drain core 11, preferably by stapling. In a preferred embodiment, the method further comprises adding an overburden layer to the soil to increase the rate of water released from the soil.

In other embodiments, the method further comprises using electroosmosis to increase the rate of water released from the soil. Accordingly, the vertical drain 1 may further comprise one or more electrically conductive strips such as, for example, copper wire attached or embedded along at least a part of the length of the drain core 11. In a preferred embodiment, the method further comprises using vacuum consolidation before, at the same time, or after operating the suction pump 9. In some embodiments, the method further comprises pumping the fluid into a horizontal drain.

In some embodiments, the suction tube 5 is perforated. One method of installing the perforated suction tube 5 into the vertical drain 1 according to the present invention includes: forming an opening in the form of a slit or cut in the filter jacket 2; inserting the suction tube 5 through the opening into the filter jacket 2, sealing the opening in the filter jacket 2; and subsequent to inserting the suction tube in the filter jacket 2, installing the vertical drain 1 into a portion of the soil to be consolidated. In a preferred embodiment, the opening in the filter jacket 2 is substantially 1.5 m from the distal end 3 of the filter jacket 2. In another preferred embodiment, the opening is sealed with tape.

The opening into which the suction tube 5 is inserted into the filter jacket 2 is located at any point along the length of the vertical drain 1. The opening is preferably located in a side wall of the filter jacket 2 or can leave the filter jacket 2 out of an open proximal end 4 of the filter jacket 2.

Some embodiments of the present invention show that the suction tube 5 may be perforated. The perforated portion of the suction tube 5 terminates inside the filter jacket 2. It is not necessary for the entirety of the non-perforated portion of the suction tube 5 to be inside the filter jacket 2.

In a preferred embodiment, the method further comprises perforating a tube to form the perforated suction tube 5. In another embodiment, the method further comprises installing the vertical drain 1 with a mandrel. In a further embodiment, the method comprises installing the anchor plate 10 at the distal end 3 of the jacket 2.

In some embodiments, the suction tube 5 is perforated at regular intervals along at least part of its length. This means that the fluid collected in the vertical drain 1 may be extracted from the vertical drain 1 at several locations, including the distal end 3, which increases the speed of the consolidation process. In other embodiments, the suction tube 5 may have no perforations or have perforations at any other spacing interval and on any portion of the suction tube 5 located within the filter jacket 2. In some embodiments, a portion of the suction tube 5 may extend beyond the filter jacket 2.

The suction tube diameter is between 5 mm and 50 mm, 6 mm and 20 mm and is preferably 10 mm. The suction apertures are distributed along the length of the suction tube 5 randomly, equi-spaced from one another or any other form of distribution. Preferably, the length of tube with suction apertures is between 0.3 m and 4 m and more preferably between 1 m and 2 m. The distance between any suction apertures in the tube 5 and the opening in the filter jacket 2 is at least between 0.3 m and 1 m, preferably 0.5 m.

In a preferred embodiment, the vertical drains 1 are arranged in an array, each connected to the pumping system. In another preferred embodiment, each vertical drain 1 comprises an array of suction tubes 5.

In a preferred embodiment, the drain core 11 is extruded from a plastic material such as polypropylene, polyethylene or other extrudable plastics. In some embodiments, the drain core 11 and/or filter jacket 2 and/or suction tube 5 are integrally formed, formed separately, or simply connected.

When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. 

1. A vertical drain for consolidating soil, the drain comprising: an elongate surrounding filter jacket having a proximal end and a distal end; and a suction tube having at least one suction aperture located within the jacket, wherein, in use, a suction pump connected to the suction tube is operable to pump flowable material through the suction tube.
 2. The vertical drain of claim 1, wherein the at least one suction aperture is located substantially at the distal end of the jacket.
 3. The vertical drain of claim 1, wherein the at least one suction aperture comprises at least one of: an open end of the suction tube; and a perforated portion of the suction tube.
 4. The vertical drain of claim 1, wherein the at least one suction aperture comprises a perforated portion of the suction tube and wherein the suction tube is perforated at substantially 100 mm intervals along at least a part of the length of the tube.
 5. The vertical drain of claim 1, wherein the at least one suction aperture comprises a perforated portion of the suction tube and wherein the suction tube is perforated not more than 1 m from the distal end of the filter jacket.
 6. The vertical drain of claim 1, further comprising a drain core within the filter jacket to provide a support structure.
 7. The vertical drain of claim 1, wherein there is an opening in the filter jacket to receive a length of the suction tube.
 8. The vertical drain of claim 1, wherein there is an opening in the filter jacket to receive a length of the suction tube and wherein a length of the suction tube is perforated to provide the at least one suction aperture and is located inside the filter jacket, and a length of the suction tube without a suction aperture passes through the opening in the filter jacket and is located outside the filter jacket.
 9. The vertical drain of claim 1, wherein the vertical drains are in an array.
 10. The vertical drain of claim 1, further comprising an anchor plate.
 11. A method of consolidating soil, the method comprising: installing a vertical drain into the soil to be consolidated, the vertical drain comprising an elongate surrounding filter jacket having a proximal end and a distal end and a suction tube having at least one suction aperture located within the jacket; and operating a suction pump connected to the suction tube to pump fluid and/or flowable material that permeates through the filter jacket from the surrounding soil through the suction tube.
 12. The method of claim 11, wherein at least one suction aperture is located substantially at the distal end of the filter jacket and at least 0.25 m from the proximal end of the jacket.
 13. The method of claim 11, further comprising securing at least a portion of the suction tube such that at least one suction aperture is located substantially at the distal end of the filter jacket.
 14. The method of claim 11, further comprising at least one of: adding an overburden layer to the soil; using electro-osmosis; and using vacuum consolidation to increase the rate of water released from the soil.
 15. A method of installing a perforated suction tube into a vertical drain, the vertical drain comprising an elongate surrounding filter jacket having a proximal end and a distal end, and the method comprising: forming an opening in the filter jacket; inserting at least a portion of the suction tube through the opening; subsequent to inserting at least a portion of the suction tube, sealing the opening in the filter jacket; and subsequent to inserting and securing the suction tube, installing the vertical drain into an area of soil to be consolidated.
 16. The method of claim 15, further comprising: perforating a tube to form the perforated suction tube, the tube having two ends; and sealing one end of the suction tube.
 17. The method of claim 15, wherein installing the perforated suction tube further comprises securing at least a portion of the suction tube inside the jacket such that at least one suction aperture is located substantially at the distal end of the filter jacket.
 18. The method of claim 15, further comprising perforating a tube to form the perforated suction tube, wherein the tube is perforated at substantially 100 mm intervals along at least a part of the length of the tube and wherein the suction tube is perforated not more than 1 m from the distal end of the filter jacket.
 19. The method of claim 15, wherein the opening in the filter jacket is formed substantially 1.5 m from the distal end of the filter jacket.
 20. The method of claim 15, wherein the vertical drain further comprises a drain core and the method further comprises securing the tube to the drain core. 